This invention relates to color projection systems, and more particularly relates to such systems employing display devices operating in the reflective mode.
Color projection systems such as for color television enable the production of image displays much larger than that reproduced on the display device itself. For example, in conventional three-tube color projection television sets, a full color image 40 inches or more in size is produced by superimposing separate red, blue and green images from three monochrome cathode ray tube (CRT) display devices, each 7 to 9 inches in size. Newer projection television sets use even smaller (e.g., 2 inch) liquid crystal display (LCD) devices operating in the transmissive mode in place of the CRTs. However, the overall optical efficiency of such LCD systems is typically on the order of only about two percent, making it difficult to design systems having acceptable brightness.
In designing a projection system using display devices operating in the reflective mode, the typically relatively small angle between the incident and reflected beams dictates a relatively long optical path in order to adequately spatially separate these beams, leading to a complex geometry for the optical system. Using three display devices, one for each of the primary colors red, blue and green, to form a full color image, significantly increases the complexity of such a system.
In U.S. Pat. No. 4,969,730, a beam separating element based on the principal of total internal reflection of one of the beams, is employed in a color projection system employing LCDs operating in the reflective mode. In this system, a source beam is split into red, blue and green beams by a dichroic cross, which also directs the split beams to the LCDs and recombines them into a single beam for projection after reflection. The beam separating element enables a spatial separation of the source beam from the projection beam.
There are several disadvantages to such an arrangement. One such disadvantage is that the dichroic cross is a relatively expensive element and has poor optical performance. One reason for this poor performance is that the incident and reflected beams strike each dichroic filter element (41, 42) of the cross (40) at different angles (see FIG. 2 of U.S. Pat. No. 4,969,730). Since performance of the filters varies with angle of incidence, and the angular relationship between the dichroic cross and the LCDs is fixed, there is no opportunity to optimize the angle of incidence of the beams on the filter elements of the cross. Another reason is that each filter element is used for both separation and recombination of the light beams. Thus, there is no opportunity to optimize the filter design for either separation or recombination, for example, to adjust spectral shaping. Another disadvantage is that by placing the beam separation element a significant distance away from the LCDs in order to accommodate the dichroic cross, it must be relatively large in size, and thus relatively costly.
A newer type of display device, the so-called deformable mirror device or DMD, is now being considered for projection applications. The DMD is a solid state device fabricated from a single piece of silicon, and comprising a matrix array of deformable mirror elements, each of which can be made to tilt in response to an applied voltage, and thus to direct reflected light into or out of an optical projection system. Using a matrix of row and column electrodes, the individual mirror elements can be made to tilt "on" or "off" in response to a video signal, to thereby re-create the video image for projection. See, for example, U.S. Pat. Nos. 4,638,309, 4,680,579 and 5,097,544.
In designing a projection system using such DMDs, a long optical path length is required due to the relatively small angular separation between the incident and projected beams.
It is possible, due to the fast response times of the mirror elements to the applied voltages, to form a full color image using a single DMD, by sequentially addressing the red, blue and green video fields at a sufficiently rapid rate that the eye integrates these sequential images into a full color image. However, such an approach reduces light efficiency by a factor of three, since only one color can be presented to the screen at a time.